We cloned the novel inhibitory receptor B and T Lymphocyte Attenuator (BTLA) in a previous cycle of this Program Project grant, and established that it is an important general regulator of immune responses. By generating and examining BTLA knockout mice, we found that BTLA regulates immune responses in several model systems, including several models of autoimmunity. We also recognized and described polymorphisms in BTLA in the mouse that influence its expression and distribution. Importantly, polymorphisms of BTLA in humans were recently linked to susceptibility to Rheumatoid arthritis, highlighting the importance of fully understanding the mechanism of BTLA action. Regulation of the intensity and the duration of immune responses is a critical for balancing the protective responses against pathogens with damage to tissues that are their result. Inhibitory receptors such as CTLA- 4, PD-1 and BTLA can contribute to this balance by ensuring that appropriate thresholds exist for activating the immune response, and by limiting the intensity and duration of responses that are initiated. However, there are still many basic issues about BTLA that are unresolved. Recent findings by our lab have identified a wholly unexpected and unprecedented interaction between BTLA, a member of the CD28/B7 Ig super family of receptors, with Herpesvirus entry mediator (HVEM), a TNF receptor family member. Our identification of HVEM as the BTLA ligand arose as part of our previous Aims to clone the ligand for BTLA, and this finding has now been confirmed by others. It is important to note that the binding between BTLA and HVEM occurs in the mouse and the human systems. While BTLA may be inhibitory, HVEM can provide activating, pro-survival signals. In fact, our recent findings and preliminary data indicate evidence for a bi- directional signaling, which has potential for delivering both inhibitory and pro-survival signals. Thus, the current application is aimed at dissecting both directions of the BTLA-HVEM interaction so that its contribution to human autoimmunity such as Rheumatoid arthritis can be understood at the basic level. Our aims are: Aim 1. Discriminate cell-intrinsic from cell-extrinsic actions of BTLA in distinct models of immune response and autoimmunity. Aim 2. Analyze the cytoplasmic signaling motifs in BTLA. Aim 3. Determine the bidirectional interactions between BTLA, LIGHT, and HVEM for signaling. Project Narrative: Autoimmunity is a major problem of health and human disease, which result from various modes of failure of the tolerance mechanisms of the immune system. Autoimmune diseases have many kinds of manifestations, such as immune responses that target the joints (arthritis), the endocrine cells of the pancreas (diabetes) or the filtering mechanism of the kidney (Lupus). Our study is directed at analyzing the basic mechanisms that control the normal activation and inhibition of the immune response, and are focused on a new gene, B and T Lymphocyte Attenuator (BTLA), that we discovered and cloned as a part of previous cycles of this same grant. Importantly, BTLA has recently been reported to be involved in the susceptibility in humans to rheumatoid arthritis. Thus, our proposed studies into this newly identified but poorly understood molecule are particularly timely.